A Gamma Ray Burst is something powerful enough to kill all life in systems not only in the system of the star, but in the surrounding region. They don't occur much, but galaxies have so many stars in their center regions that it is unlikely for life to exist there due to Gamma Ray Bursts. This eliminates 90% of all stars as places where life like ours would evolve.
Our best defense against these would be to spread out into multiple systems. We can even colonize the most dense parts of the galaxy, but it would come at the cost of periodically losing the entire populations of star systems.
Couple this with this article about red dwarfs, and now probably 95% of all star systems are uninhabitable.